Projection type image display apparatus

ABSTRACT

A compact, light projection type image display apparatus based on reflection image display devices which provide high brightness without contrast deterioration caused by light leakage associated with black image display. The apparatus uses, as polarizers/analyzers for reflection image display devices, reflection polarizing plates which function as polarizing plates by their grating function only in a specific direction. In each light path, the reflection polarizing plate is located just before/after the reflection image display device.

The present application is a continuation of U.S. application Ser. No.10/648,788, filed on Aug. 27, 2003 now U.S. Pat. No. 6,893,131.

BACKGROUND OF THE INVENTION

The present invention relates to a projector which projects an image ona screen using reflection liquid crystal panels such as a reflectionliquid crystal projector, a projection type image display apparatus suchas a reflection liquid crystal rear projector, and a projection typeimage display apparatus optical unit.

An optical unit for a reflection liquid crystal projector which usesreflection liquid crystal panels is explained next. Non-polarized lightcoming from a light source is linearly polarized by a polarizationconverter and cast on a polarizer. The unwanted polarized component isremoved from the light by the polarizer before it reaches a reflectionliquid crystal panel. The light is modulated by the reflection liquidcrystal panel so that its polarization state is changed on apixel-by-pixel basis according to image signal. Then, it reaches ananalyzer. The quantity of light which is transmitted or reflected by theanalyzer is determined according to the polarization state of the lightemitted from each pixel. An image thus obtained is projected in anenlarged form by a projection lens. A polarizing beam splitter prism(hereinafter called a PBS prism) is commonly used as apolarizer/analyzer. A PBS prism has a dielectric multilayer film(hereinafter called a PBS film) which transmits p-polarized light andreflects s-polarized light.

In a reflection type liquid crystal projector which uses a PBS prism asa polarizer/analyzer, light leakage from the PBS prism occurs for thefollowing reason when a black image is displayed. The s- andp-polarization directions of light rays which are not parallel to themain incidence (entrance) plane are different between when they aretransmitted or reflected by the PBS prism and when they reenter the PBSprism after being reflected by the reflection liquid crystal panel.However, after being transmitted or reflected by the PBS prism, raysretain their polarization direction when they reenter the prism.Therefore, rays parallel to the main incidence plane are completelytransmitted or reflected when they reenter the prism; on the other hand,for rays which are not parallel to the main incidence plane, thes-polarized component is reflected and the p-polarized component istransmitted. For this reason, rays which are not parallel to the mainincidence plane may cause light leakage, resulting in black imagecontrast deterioration.

Also, a contrast improvement method which uses a quarter-wave platebetween a projection lens and a cross dichroic prism has been suggested.

However, in the case that a PBS prism is used as a polarizer/analyzerfor a reflection type liquid crystal panel as mentioned above, even whena quarter-wave plate is disposed just before the panel in order toprevent contrast deterioration, the effect of the quarter-wave plate isnot satisfactory. A wave plate has specific wavelength and anglecharacteristics. As the difference of the incident light wavelength fromthe design center wavelength increases or the incidence angle increases,the performance of the wave plate decreases, so light leakage cannot beprevented completely and contrast deterioration will occur. Even when apolarizing plate is disposed between the PBS prism and the projectionlens for the purpose of preventing leak light from being projected onthe screen, it is impossible to prevent light leakage completely becausethe leak light includes rays polarized in the same direction as thedirection of the polarizing plate transmission axis.

Besides, the use of the PBS prism is disadvantageous from the view pointof weight. Furthermore, when the PBS prism uses a glass material with alow photoelastic coefficient to avoid birefringence which might causecontrast deterioration, it may be relatively heavy because of therelatively large specific gravity of the glass material, and costlybecause the material is not widely available on the market.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above problems andprovides a light, compact reflection type image display apparatus whichuses reflection image display devices to deliver satisfactory imagequality performances such as brightness, high contrast, and highresolution.

In order to solve the above problems, the invention provides aprojection type image display apparatus which comprises the following: alamp unit which emits light; three reflection image display devices aslight valve means which make an optical image according to an imagesignal; a color separation system which separates light from the lampunit into three color light components and lets them impinge on thecorresponding color reflection image display devices; a color combiningsystem which combines the three color light components from thereflection image display devices; and a projection means which projectsa color-combined optical image. In addition, the apparatus has: flatreflection polarizing plates as polarizers/analyzers for the reflectionimage display devices which, by their diffraction function, reflectprimary light polarized in a specified direction and transmit secondarylight polarized in a direction virtually perpendicular to the specifieddirection; and auxiliary analyzers which transmit either the primarypolarized light or the secondary polarized light on the exit side of thereflection polarizing plates. Here, the color combining system consistsof a color combining prism and three color light components resultingfrom color separation by the color separation system reach thereflection image display devices through the reflection polarizingplates and the reflected light which constitutes an optical image madeby the reflection image display devices enters the color combiningsystem through the reflection image display devices.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more particularly described with reference to theaccompanying drawings, in which:

FIG. 1 shows the general structure of a projection type liquid crystaldisplay apparatus according to a first embodiment of the presentinvention;

FIG. 2 is a graph showing the relation between the reflection polarizingplate surface form and pixel convergence errors, as a result of lightray tracking simulation;

FIG. 3 shows the general structure of a projection type liquid crystaldisplay apparatus according to a second embodiment of the presentinvention;

FIG. 4 shows the general structure of a projection type liquid crystaldisplay apparatus according to a third embodiment of the presentinvention; and

FIG. 5 shows the general structure of a projection type liquid crystaldisplay apparatus according to a fourth embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Next, preferred embodiments of the present invention will be described,referring to the accompanying drawings.

FIG. 1 shows a reflection liquid crystal projector optical unitaccording to an embodiment of the present invention. A projection typeimaged is play apparatus 22 has a lamp unit (light source) 1 whichincorporates a lamp 1 a. The lamp 1 a is a white lamp such as aultra-high pressure mercury lamp, metal halide lamp, xenon lamp, mercuryxenon lamp or halogen lamp. Light emitted from the lamp 1 a in the lampunit 1 is condensed and reflected by an ellipsoidal, parabolic oraspheric surface reflector 1 b. Since the lamp unit 1 becomes hot due tothe heat of the lamp 1 a, it is cooled by a cooling fan 40 behind it.

The light coming from the lamp unit 1 impinges on a first lens array 32.The first lens array 32 consists of plural condenser lenses disposed ina rectangular frame which has the almost same size as the exit openingof the reflector 1 b. It condenses the light from the lamp unit 1 andmakes plural secondary lamp images. Then, the light from the first lensarray 32 passes through a second lens array 33. The second lens array33, which consists of plural condensers, is located near the spot wherethe plural secondary lamp images are made. The second lens array 33focuses individual lens images of the first lens array 32 on reflectionliquid crystal panels 10. The first lens array 32 and the second lensarray 33 have vertically long cells to bend the light path vertically sothat light will be later cast on reflection image display devices. Thelight emitted from the second lens array 33 enters a polarizationconverter 31 which consists of an array of rhomboidal prisms. Therhomboidal prisms are disposed so as to match the vertical pitch of theoptical axes of the lenses of the second lens array 33 and the size ofeach prism is almost one half of the lens width.

There is a polarization/separation film on the prism surface. Theincident light is separated into p-polarized light and s-polarized lightby this polarization/separation film. The p-polarized light passesthrough the polarization/separation film and leaves the prism. On theother hand, the s-polarized light is reflected by thepolarization/separation film and again reflected towards the originaloptical axis inside an adjacent rhomboidal prism; then it is rotated 90degrees to be converted into p-polarized light by a half-wave platedisposed on the exit plane of the prism before leaving the prism. Acollimator lens 34 has a positive refractive power and provides alight-condensing function. The light is cast on three color (R, G, andB) reflection liquid crystal panels 11.

This structure has two stages. The light is color separated by anRB-transmission G-reflection dichroic mirror 36 into RB light and Glight. Then, the G light is reflected upwards by a white reflectionmirror 5. An R-transmission B-reflection dichroic mirror 37 transmits Blight and reflects R light upwards to separate the RB light into R lightand B light. The B light passes through lenses 4 and reaches thereflection mirror 5 where it is reflected upwards. The separated R, Gand B light rays in the three light paths respectively pass through anauxiliary polarizer for R (not shown), an auxiliary polarizer for G 92,and an auxiliary polarizer for B 93; then through a reflectionpolarizing plate for R (polarization separator) 101, a reflectionpolarizing plate for G (polarization separator) 102, and a reflectionpolarizing plate for B (polarization separator) 103 before reaching areflection liquid crystal panel for R 111, a reflection liquid crystalpanel for G 112, and a reflection liquid crystal panel for B 113.

Reflection polarizing plates 10 are almost rectangular and their shortsides are inclined approx. 45 degrees with respect to the optical axis.The distance from each reflection liquid crystal panel 11 to thecorresponding reflection polarizing plate 10 can be shortened and thusthe back focus of the projection lens can be shortened. This mitigatesastigmatism in the projection lens.

Each reflection liquid crystal panel 11 has as many liquid crystaldisplay areas with an aspect ratio of 16:9 as pixels to be displayed(for example, 1,900 pixels in the horizontal direction and 1,080 pixelsin the vertical direction). The phase angle of polarized light in eachpixel on the panel 11 is varied according to externally driven signal.Light whose polarization direction is matched is analyzed by areflection polarizing plate and an auxiliary analyzer. Regarding lightwhose polarization direction is half-matched, the quantity of light tobe analyzed is determined depending on the degree of polarization of thereflection polarization plate and auxiliary analyzer. An image isdisplayed according to external signal in this way. For black display ofthe reflection liquid crystal panels 11, the polarization direction isvirtually the same as the direction of incident light and the light goesback along the incident light path to the lamp.

In each color light path, the optical axis of the reflection plane ofeach reflection liquid crystal panel 11 is virtually perpendicular tothe optical axis of each color light entrance (incidence) plane of across dichroic prism 14 and in the vicinity of the spot where bothoptical axes cross virtually perpendicularly, there is a reflectionpolarizing plate 10 (inclined approximately 45 degrees with thecorresponding optical axis). In this structure, optical components (fromthe reflection polarizing plate to the auxiliary analyzer) disposed inthe color light paths of the cross dichroic prism 14 are symmetrical andalmost equal among the light paths when viewed from the entrance planeof the cross dichroic prism 14. From the viewpoint of image focusing,the light path lengths from the panels 11 to the projection lens 15 mustbe almost equal, so this symmetrical disposition minimizes interferencebetween the components or the light paths, which makes design work easy.Also, space can be efficiently used and the unit size can be reduced.

The rays reflected by the R, G, and B light reflection liquid crystalpanels are respectively reflected by R, G, and B light reflectionpolarizing plate 101, 102, 103 and rotated approximately 90 degrees andtransmitted by R, G, and B light auxiliary analyzers 121, 122, 123; thenG rays are transmitted by a half-wave plate for G 13 to be convertedinto p-polarized light before all R, G and B rays enter the crossdichroic prism 14.

The reflection plane optical axes of the three reflection liquid crystalpanels 11 are oriented in the same way and the three reflection planesare almost equal in height with one of them as the reference plane. Thismakes it possible to make pixel position adjustments of the three panels11 and fit the panels 11 to the respective panel holders after theadjustments, using a special tool (jig) in the same direction moreeasily. Therefore, working time is shortened and the cost is reduced. Ifa tool has a rotary mechanism, all the panels 11 can be adjusted usingit. This also implies reduction in the tool designing and manufacturingcost.

Since there is no light path on the rear (lower) side of each of thereflection liquid crystal panels, a radiation board may be attachedthereon to cool the panel. In this embodiment, one cooling fan (notshown) disposed on the upper side may be sufficient to cool the threereflection liquid crystal panels efficiently.

The three reflection liquid crystal panels 11 are located near and abovethe upper side of the cross dichroic prism 14 which is not used as alight path. This prevents interference between the panels 11 or thepanel holders and the cross dichroic prism 14 and allows the use of asmaller optical engine.

The reflection polarizing plates 10 are fixed in position with theirreflection surfaces held against three protrusions on the chassis bymeans of leaf springs, etc. in the opposite direction. The angle of thereflection plane surfaces of the reflection polarizing plates 10 whichreflect light coming from the panels must be strictly controlled becauseit affects the image position on the screen. Since this chassis ismanufactured by molding, it is easy to control the positions and shapeof the three protrusions with high accuracy in the chassis massproduction process. The reflection plane angle can be controlledaccurately by pushing and holding the reflection plane surfaces directlyagainst these three protrusions.

In this embodiment, the surface forms of the R, G, and B lightreflection polarizing plates 101, 102, and 103 are such that thedifference between one of them (reference plate) and the other plates iswithin ±3 (λ/inch). Light which has exited a reflection liquid crystalpanel 11 is reflected by a reflection polarizing plate 10. If areflection polarizing plate is concave or convex, the plate functions asa lens and affects the image quality on the screen. For this reason, thesurface forms of the reflection polarizing plates 10 must be controlled.For a structure which uses three such panels, it is particularlyimportant to use one polarizing plate as a reference and minimize thesurface form differences of the other two plates from the referenceplate in order to assure a high image focusing performance. FIG. 2 showsthe relation between the reflection polarizing plate surface form andpixel convergence errors, as a result of result of light ray trackingsimulation. The graph suggests that the plate surface form differenceshould be within ±4 (λ/inch) in order to keep pixel convergence errors0.3 pixel or less. Preferably, the difference should be within ±3(λ/inch); in order to avoid image quality deterioration, it should be atleast within ±4 (λ/inch). If the surface is not flat, the projectionlens focus adjusting function can be used to compensate for it to someextent. Alternatively, for the purpose of facilitating componentmanagement and selection, it is also acceptable that all the threereflection polarizing plates have convex or concave reflection surfaces.

Generally, chromatic aberration (image focusing performance index)depends on wavelength and this index becomes better or worse in theorder of R, G, and B. In other words, G always takes the center value.Hence, it is reasonable to use the reflection polarizing plate for G asa reference and keep the differences from the reference within Newton ±3λ.

Regarding the auxiliary polarizers 9, reflection polarizing plates 10,and auxiliary analyzers 12, as the contrast ratio is increased, thetransmittance is decreased, and vice versa. This implies a tradeoffbetween contrast and brightness in terms of projection type imagedisplay apparatus performance. Not a single component but a plurality ofcomponents are used to assure satisfactory contrast: the auxiliarypolarizers 9, reflection polarizing plates 10, and auxiliary analyzers12. In accordance with the rule mentioned below, these opticalcomponents may be combined so as to assure high efficiency and highcontrast in the projection type image display apparatus.

The contrast ratio of an optical system is calculated from the followingformula:1/(optical system contrast ratio)=1/(optical system contrast ratio onthe panel entrance side)+1/(optical system contrast ratio on the panelexit side)

This formula indicates that brightness and contrast cannot be improvedefficiently if only the optical system contrast ratio on the panelentrance or exit side is improved. Balancing between the entrance andexit side contrast ratios is the best way to optimize both brightnessand contrast. The optical system contrast ratio is calculated as theproduct of contrast ratios of components. When the contrast ratio of theauxiliary polarizers is expressed by A; that of the auxiliary analyzersby D; and the transmission contrast ratio and reflection contrast ratioof the reflection polarizing plates by B and C, respectively, theentrance side optical system contrast ratio is calculated by A*B whilethe exit side optical system contrast ratio is calculated by C*D.Therefore, for the purpose of balancing, the auxiliary polarizers,auxiliary analyzers, and reflection polarizing plates should meet thefollowing relation: A*B=(0.1–10)*C*D. When no auxiliary polarizers oranalyzers are used, the above relation should be met where 1 should besubstituted for their contrast ratio.

Next is a method of measuring the contrast ratio of an absorptionpolarizing plate which is used as an auxiliary polarizer/analyzer. Ameasuring light source emits light. Due to an aperture behind the lightsource, the divergence of the beam which reaches the object to bemeasured is expressed as nearly F20. Located after the light source is ameasuring polarizing plate whose transmission axis is aligned totransmit p-polarized light for characteristic measurement of p-polarizedlight or s-polarized light for characteristic measurement of s-polarizedlight. The object to be measured is located after it to make ameasurement. The light passes through the object and reaches a measuringlight receiver where the spectral distribution of the transmitted lightis measured. The transmittance for the transmission axis of theabsorption polarizing plate is measured in two modes: an s-polarizedlight transmittance measuring mode and a p-polarized light transmittancemeasuring mode. Brightness is calculated by multiplying the spectraldistribution of measured transmittances by spectral luminous efficacy.Hence, theoretically brightness is calculated by wavelength integration∫T(λ)*A(λ)dλ where transmittance and spectral luminous efficacy in thewavelength band used are respectively expressed by T(λ) and A(λ). Sinceactually measured transmittance T(λ) values are not continuous,brightness is calculated by summation of T(λ)*A(λ)dλ in the wavelengthband used. In the case of reflection, reflectance R(λ) should be usedinstead of transmittance T(λ). When the absorption polarizing plate isadjusted for transmission of p-polarized light, the contrast ratio iscalculated as (brightness of p-polarized light)/(brightness ofs-polarized light). When the absorption polarizing plate is adjusted fortransmission of s-polarized light, the contrast ratio is calculated as(brightness of s-polarized light)/(brightness of p-polarized light).

Next is a method of measuring the contrast ratio of a reflectionpolarizing plate. The transmission axis of the reflection polarizingplate is inclined 45 degrees with respect to the optical axis forp-polarized light. Measurements are made to obtain spectraldistributions concerning transmission and reflection of incidents-polarized and p-polarized light. Since the transmission axis isadjusted for p-polarized light, the transmission contrast ratio isexpressed by (brightness of p-polarized light)/(brightness ofs-polarized light) and the reflection contrast ratio by (brightness ofs-polarized light)/(brightness of p-polarized light).

Generally speaking, regarding the reflection polarizing plate, itstransmission contrast ratio of p-polarized light is higher than itsreflection contrast ratio of s-polarized light. Hence, if the contrastratio of the auxiliary analyzers is higher than the contrast ratio ofthe auxiliary polarizers, higher efficiency and higher contrast can beachieved. In other words, when the transmittance of the auxiliaryanalyzers is lower than that of the auxiliary polarizers, higherefficiency and higher contrast can be achieved.

The display area of the reflection liquid crystal panels 11 is supportedby a structure (not shown) which is asymmetric as viewed from the centerof the display area. The reflection liquid crystal panels 11 aredisposed so that the short side of the structure is nearer to the crossdichroic prism 14 as viewed from the center of the display area. Thismakes it possible to dispose the reflection liquid crystal panels 11nearer to the cross dichroic prism 14, so the apparatus size can bereduced and the distance from the reflection liquid crystal panels 11 tothe cross dichroic prism 14 can be shortened. Consequently, the backfocus of the projection lens 15 can be shortened and astigmatism can bemitigated.

In another embodiment, on the entrance side of the reflection polarizingplates, there is a polarization/separation prism 38 which reflectss-polarized light (virtually linearly polarized light), and transmitsp-polarized light (virtually linearly polarized light) which isvirtually perpendicular to it. Unlike the film auxiliary polarizers 9,the polarization/separation prism 38 is heat-resistant and eliminatesthe need for cooling by a cooling fan, so noise is reduced.

Next, a second embodiment of the invention will be described referringto FIG. 3.

The portion from the lamp unit 1 to the collimator lens 34 is the sameas in the embodiment shown in FIG. 1 except that the first lens arrayand the second lens array have horizontally long cells and apolarization converter 31 is disposed in a way to match vertical pitchesand the light path is bent approximately 90 degrees by the reflectionmirror 5.

Light is separated into R (red), G (green), and B (blue) light bydichroic mirrors 6 and 7. Then, B light rays are bent 90 degrees by aB-reflection mirror 17. R, G, and B light respectively pass through R,G, and B light auxiliary polarizers 91, 92, 93, then through R, G, and Blight reflection polarizing plates 101, 102, 103 and respectively reachR, G, and B light reflection liquid crystal panels 111, 112, 113. Therays reflected by the R, G, and B light reflection liquid crystal panelsare reflected and bent 90 degrees by the R, G, and B light reflectionpolarizing plates 101, 102, and 103 respectively, then transmittedthrough R, G, and B light auxiliary analyzers 121, 122, 123. G light istransmitted by a half-wave plate for G 13 and converted into p-polarizedlight and all R, G, and B light rays enter the cross dichroic prism 14.R, G, and B light rays are combined into white and projected on thescreen in enlarged form by the projection lens 15.

In this embodiment, contrast is improved with respect to obliqueincident light by a viewing angle compensation retardation film (notshown) located just before a reflection liquid crystal panel.

An explanation of a reflection polarizing plate is given below. Areflection polarizing plate, which functions as a polarizing plate byits grating function in a specific direction only, reflects polarizedlight parallel to the grating and transmits polarized lightperpendicular to the grating. The contrast ratio is almost equal inregard to any light rays on a plane including the polarizing plate'stransmission axis and its normal and on a plane including the polarizingplate's absorption or reflection axis and its normal. This structure hasno possibility of light leakage which might occur in a structure with aPBS prism before or after the panels, and high contrast is achieved withno additional component such as a quarter-wave plate. Most leak lightfrom the PBS prism is polarized in the same direction as the directionof the transmission axis of the polarizing plate, so it would beimpossible to prevent light leakage completely even when a polarizingfilm (analyzer) is placed between the projection lens and the reflectionliquid crystal panel. On the other hand, when a reflection polarizingplate is employed instead, light leakage is often caused by a lowcontrast ratio of the reflection polarizing plate; therefore, the use ofan auxiliary polarizer and an auxiliary analyzer in such a situationprevents most of light leakage, leading to a higher contrast.

A cooling fan 40 cools auxiliary polarizers 9, reflection polarizingplates 10, reflection liquid crystal panels 11 and auxiliary analyzers12. Since the auxiliary polarizers 9 and the auxiliary analyzers 12 areboth made of an absorptive film material, they must be cooled down toapproximately 70 degrees Celsius or less. In black display, theauxiliary analyzers 12 receive unwanted light which has been onceintercepted by the reflection polarizing plates 10 located on theirentrance side, and thus absorb little unwanted light. By contrast, theauxiliary polarizers 9 directly receive light which contains unwantedlight and thus absorb more unwanted light and generate more heat. Forthis reason, the auxiliary polarizers 9 must be more cooled than theauxiliary analyzers 12. The cooling fan is positioned or its air duct isdesigned so that cooling air blows towards the auxiliary polarizers 9more strongly than towards the auxiliary analyzers 12

An exit quarter-wave plate 35 is attached to the exit plane of the crossdichroic prism 14. Since the exit quarter-wave plate 35 is to be locatedin the combining light path, it should be a wideband quarter-wave plate.The angle θ of the lag axis 35 a of the exit quarter-wave plate 35 withrespect to the absorption axis 12 a of the corresponding auxiliaryanalyzer 12 should be almost within the range of 40 to 50 degrees or −40to −50 degrees. In this embodiment, the angle is set to approximately 45degrees. If the contrast ratios of the auxiliary polarizers 9 andreflection polarizing plates 10 are insufficient and the contrast ratiofor incident light on the reflection liquid crystal panels 11 is low, orthe phase difference given by the reflection liquid crystal panels 11for black display is inadequate, the light emitted from the panels 11for black display contains s-polarized light which cannot be interceptedby the reflection polarizing plates 10 and auxiliary analyzers 12.s-polarized light passes through the reflection polarizing plates 10 andauxiliary analyzers 12 and impinges on the projection lens 15. Theprojection lens 15 consists of many lenses and its total transmittanceis 85% or so, which means that it reflects 15% of light which itreceives. If no exit quarter-wave plate 35 is provided, this reflectedlight enters as virtually s-polarized light which is inclined 90 degreesfrom virtually p-polarized light which the reflection liquid crystalpanels 14 are to receive. For black display on the panels, outgoinglight, which remains virtually s-polarized, cannot be intercepted by thereflection polarizing plates 10 and auxiliary analyzers 12; it goesthrough the projection lens 15 and reaches the screen, resulting incontrast deterioration. If an exit quarter-wave plate 35 is provided asin this embodiment, virtually s-polarized leak light is transmitted bythe exit quarter-wave plate 35 with a lag axis inclined nearly 45degrees and reflected by the projection lens 15 and again transmitted bythe exit quarter-wave plate 35 while it is converted into virtuallyp-polarized light. Therefore, it is absorbed by the auxiliary analyzers12, resulting in contrast improvement.

On the cross dichroic prism 14, an exit quarter-wave plate 35 isattached to its exit plane for combined light, an auxiliary analyzer forR 121 to the entrance plane for the R light path, an auxiliary analyzerfor B to the entrance plane for the B light path, and a half-wave platefor G 13 to the entrance plane for the G light path. In other words,wave plates or polarizing plates are attached to all four light pathplanes. The cross dichroic prism 14 must be made with high precision sothat pixel convergence errors on the screen are minimized. The crossdichroic prism 14 is made by bonding four triangular prisms together.The apexes of these prisms are frangible and must be carefully handled.When all the four planes are bonded together as mentioned above, thetroublesome AR (antireflection coating) process for the prism 14 can beeliminated, leading to cost reduction. In the AR process, the prism 14is heated and residual stress occurs in it. To improve contrast,reflected light from the projection lens 15 is rotated by the exitquarter-wave plate 35 and absorbed by the corresponding auxiliaryanalyzer 12. If there should be residual stress in the prism 14,birefringence would occur and result in black unevenness. On the otherhand, if wave plates or polarizing plates are bonded to all the fourplanes, the AR process is eliminated for all the four triangular prismsand residual stress in the prism 14 is decreased and black unevenness isreduced. Because all the four prisms have the same level of residualstress, they expand almost equally as they receive light. Therefore,angle discrepancy from the initial positions on the blue and red lightreflection planes of the cross dichroic prism 14 is small. Hence, theamount of pixel convergence error that arises on each panel during useis smaller. Since the prisms are bonded together, there is lessinterface with the air, which prevents contrast deteriorationattributable to reflection on the interface. The exit quarter-wave plate35 is made of crystal. It causes less in-plane change in heat-inducedbirefringence than a film-based quarter-wave plate, thereby reducingblack unevenness.

The reflection polarizing plates 10 separate light into p-polarizedlight and s-polarized light using the effect of diffraction gratingwhose pitch is shorter than the wavelength of light. Contrast is poorerfor the B light path with relatively short wavelengths than for the Rand G light paths. So, the contrast for the B light path is improved byusing the diffraction grating pitch only for the reflection polarizingplate 10 in the B light path which is shorter than that for thereflection polarizing plate in the R light path.

In addition, for contrast balancing among the three colors and betterblack display, the auxiliary polarizers 93 and auxiliary analyzer 123for the B light path are designed to have higher contrast ratios thanthe auxiliary polarizers 91 and auxiliary analyzer 121 for the R lightpath and the auxiliary polarizers 92 and auxiliary analyzer 122 for theG light path.

Here, each auxiliary analyzer 12 has a double plate structure toincrease its heat resistance. The required cooling air flow rate islower and, therefore, the required cooling fan speed is lower, so thewind noise of the cooling fan is reduced, allowing quiet operation.

FIG. 4 shows a third embodiment of the present invention. The structureis the same as that of the embodiment illustrated in FIG. 3 except thefollowing points. The reflection polarizing plates 10, reflection liquidcrystal panels 11 and auxiliary analyzers 9 are located on the surfaceof the hermetically sealed chassis 42 which borders on the outside.Because the hermetically sealed chassis 42 are dustproof, they preventdust adhesion on the reflection polarizing plates 10, reflection liquidcrystal panels 11, etc., so that on-screen image missing or imagequality deterioration may not occur. The use of optical components suchas the reflection polarizing plates 10 and auxiliary analyzers 9 asboundaries assures air tightness without interfering with the lightpaths. Hermetic seals are placed between the optical components and thechassis 42 to assure air tightness. A cooling fan cools the auxiliaryanalyzers. Only the rear sides of the reflection liquid crystal panels11 are exposed to the outside and cooled. Light is reflected by thereflection polarizing plates 10 and cast on the reflection liquidcrystal panels 11 and reflected by them. Then the light is analyzed bythe reflection polarizing plates 10 and only its effective component istransmitted to the projection lens.

FIG. 5 shows a fourth embodiment of the present invention. The structureis the same as that of the embodiment illustrated in FIG. 3 except thefollowing points. A color separation system separates light into R, Gand B light, which pass through the auxiliary analyzers 91, 92, and 93and through the reflection polarizing plates 10 and reach the reflectionliquid crystal panels 11. The image light modulated and reflected by thereflection liquid crystal panels 11 is reflected by the reflectionpolarizing plates 10 before entering a color combining prism 14. Ahermetically sealed chassis 42 houses the reflection polarizing plates10, reflection liquid crystal panels 11, cross dichroic prism 14 andauxiliary analyzers 9 and has auxiliary polarizers 9 on its boundaries.A cooling fan cools the auxiliary analyzers 9.

Alternatively, the reflection polarizing plates 10, reflection liquidcrystal panels 11, cross dichroic prism 14 and quarter-wave plates 9 maybe located on the boundaries of the hermetically sealed chassis 42.

The long sides of the reflection polarizing plates are free andthermally expansible, so deformation due to thermal expansion isminimized and pixel convergence errors on the screen are reduced.

Although the above discussion concerns embodiments which use threereflection liquid crystal panels, the present invention is not limitedthereto but may be embodied in the form of a structure which uses one ortwo reflection liquid crystal panels, with the same effects.

As explained so far, a reflection type liquid crystal projector opticalunit or a reflection type liquid crystal projector according to thepresent invention has the abovementioned basic structure that usesreflection polarizing plates which function as polarizing plates bytheir grating function only in a specific direction, so that the needfor a PBS prism or a quarter-wave plate for PBS prism compensation iseliminated and there is no possibility of interference between theprojection lens and the structural components holding the reflectionliquid crystal panels. This brings about the effects of contrastimprovement and decrease in the number of required components(brightness improvement) without resolution deterioration, therebyrealizing a light, compact unit or projector.

The foregoing invention has been described in terms of preferredembodiments. However, those skilled, in the art will recognize that manyvariations of such embodiments exist. Such variations are intended to bewithin the scope of the present invention and the appended claims.

1. A projection type image display apparatus comprising: a light sourceunit which emits light; three reflective type image display devices aslight valve means, which form an optical image corresponding to an imagesignal; a color separation system, which separates the light from saidlight source unit into three color light components, including a redcolor light, a green color light and a blue color light, and whichguides each of the color light components to impinge upon acorresponding one of said color reflective type display devices; a colorcombining system, which combines those three color light components fromsaid reflective type image display devices; a projection lens whichprojects a color-combined optical image; flat reflective type polarizingplates, each of which transmits a first light polarized into apredetermined polarization direction and reflects a second lightpolarized into a direction nearly perpendicular to the predeterminedpolarizing direction, due to diffraction function thereof, as apolarizer and an analyzer for said reflective type image displaydevices; auxiliary polarizers, each of which transmits said firstpolarized light, being provided on an entrance side of said flatreflective type polarizing plates; and auxiliary analyzers, each ofwhich transmits said second polarized light, being provided on anentrance side of said flat reflective type polarizing plates, wherein:said color combining system is made up with a color combining prism, andis disposed, so that: each of the color light components, which areseparated by said color separation system, penetrates through saidauxiliary analyzer and said flat reflective type polarizing plates, tobe incident upon each of said reflective type image display devices; andthe optical image of reflection light, which is formed by saidreflective type image display device, reflects upon said flat reflectivetype polarizing plate and penetrates through said auxiliary analyzer, tobe incident upon said color combining system, and further, a contrast ofsaid auxiliary polarizer for blue color light, which is disposed on anoptical path of the blue color light, is made larger than those of saidauxiliary polarizers for the other color lights.
 2. The projection typeimage display as described in the claim 1, wherein the contrast of saidauxiliary analyzer for the blue color light, which is disposed on theoptical path of the blue color light, is made larger than those of saidauxiliary analyzers for the other color lights.
 3. The projection typeimage display as described in the claim 1, wherein the contrasts of saidauxiliary analyzers for the respective color light components are madelarger than those of said auxiliary polarizers for the respective colorlight components.
 4. The projection type image display as described inthe claim 1, wherein said flat reflective type polarizing plates aredisposed within an inside of a nearly hermetically closed structure, andsaid auxiliary polarizers and said reflective type image display devicesand said auxiliary polarizers build up a portion of wall surfaces ofsaid hermetically closed structure.
 5. The projection type image displayas described in the claim 1, wherein said flat reflective typepolarizing plates, said reflective type image display devices and saidauxiliary analyzers are disposed within an inside of a nearlyhermetically closed structure, and said auxiliary polarizers build up aportion of wall surfaces of said hermetically closed structure.
 6. Theprojection type image display as described in the claim 2, wherein thecontrasts of said auxiliary analyzers for the respective color lightcomponents are made larger than those of said auxiliary polarizers forthe respective color light components.
 7. The projection type imagedisplay as described in the claim 2, wherein said flat reflective typepolarizing plates are disposed within an inside of a nearly hermeticallyclosed structure, and said auxiliary polarizers and said reflective typeimage display devices and said auxiliary polarizers build up a portionof wall surfaces of said hermetically closed structure.
 8. Theprojection type image display as described in the claim 2, wherein saidflat reflective type polarizing plates, said reflective type imagedisplay devices and said auxiliary analyzers are disposed within aninside of a nearly hermetically closed structure, and said auxiliarypolarizers build up a portion of wall surfaces of said hermeticallyclosed structure.
 9. A projection type image display apparatuscomprising: a light source unit which emits light; three reflective typeimage display devices as light valve means, which form an optical imagecorresponding to an image signal; a color separation system, whichseparates the light from said light source unit into three color lightcomponents, including a red color light, a green color light and a bluecolor light, and which guides each of the color light components toimpinge upon a corresponding one of said color reflective type displaydevices; a color combining system, which combines those three colorlight components from said reflective type image display devices; aprojection lens which projects a color-combined optical image; flatreflective type polarizing plates, each of which transmits a first lightpolarized into a predetermined polarization direction and reflects asecond light polarized into a direction nearly perpendicular to thepredetermined polarizing direction, due to diffraction function thereof,as a polarizer and an analyzer for said reflective type image displaydevices; auxiliary polarizers, each of which transmits said firstpolarized light, being provided on an entrance side of said flatreflective type polarizing plates; auxiliary analyzers, each of whichtransmits said second polarized light, being provided on an entranceside of said flat reflective type polarizing plates; and a cooling fan,wherein an amount of air flow of a cooling air from said cooling fanupon said auxiliary polarizers is made larger than that upon saidauxiliary analyzers.
 10. The projection type image display as describedin the claim 9, wherein at least three set of said cooling fans areprovided for each of the color light components, and each thereof isprovided in vicinity of each of said auxiliary polarizers for the colorlight components.
 11. The projection type image display as described inthe claim 10, wherein said flat reflective type polarizing plates, saidreflective type image display devices and said auxiliary analyzers aredisposed within an inside of a nearly hermetically closed structure, andsaid auxiliary polarizers build up a portion of wall surfaces of saidhermetically closed structure.